A- Arc Flash Analysis-general

7/30/2019 A- Arc Flash Analysis-general

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ARC FLASH ANALYSIS

Main objectives in designing electrical power distribution systemsare reliability & cost effectiveness

Most common reliability criteria are:-

Power continuity by minimizing downtime & outages

Safety of personnel & equipment

Safety of personnel should be one of the primary concern of theplant owners and this should not be sacrificed at any cost.


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ARC FLASH ANALYSIS

Electrocution is the first thing that usually comes to mind when

someone is killed or injured by electricity; however, this is not theonly hazard that exists.

It is widely recognised the higher the voltage of an electrical powersystem, the greater the risk for people working on or near

energised conductors and equipment.

Although the electric shock hazard increases with voltage, anotherhazard known as electrical flashover or arc flash, can actually bemuch worse at lower voltages.

An electrical arc flash can be devastating causing severe burninjury and even death.


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ARC FLASH ANALYSIS

Apart from electric shock, electric arc flash poses serious safety

issues that need to be addressed and these are covered under thesubject of ARC HAZARD and its analysis

After years of extensive research, paper work, tests and otherevaluation work, it was recognized that

ARC FLASH posed very serious plant & personnel safetyHAZARD

and it was

ARC FLASH that injured or killed more electrical plantpersonnel than electric shocks.


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ARC FLASH ANALYSIS

In 1982, Dr RALPH LEE first identified arc flash in a IEEE-PASpaper and it was recognised that

ARC FLASH FLAH hazard study should be incorporated insubstation design work as a complement to short circuitanalysis and relay setting/relay coordination work


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ARC FLASH ANALYSIS

Arising out of these researches, various safety committeesworldwide such as OSHA, NFPA & IEEE have formulated rules forensuring that the electrical equipment and installation engineeringare adequate to:-

Prevent ARC FLASH HAZARDS

and

Protect personnel from ARC FLASH HAZARDS


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ARC FLASH ANALYSIS

Finally in 2002, IEEE 1584 was prepared which established therules that we need to now follow in switchgear/ protection designand PPE category requirement

In US & EUROPE and various countries heavy penalties apply forfirms not following the IEEE, NFPA rules and for Owners whobelieve in making huge returns at the risk of their personnel safety.


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ARC FLASH ANALYSIS

Compliance with OSHA involves adherence to a plan summarisedbelow:

A facility must provide, and be able to demonstrate, a safetyprogram with defined responsibilities.

Calculations for the degree of arc flash hazard.

Correct personal protective equipment (PPE) for workers.

Training for workers on the hazards of arc flash.

Appropriate tools for safe working.Warning labels on equipment.


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ARC FLASH ANALYSIS

Note that the labels are provided by the equipment owners, not themanufacturers. It is expected that the next revision of the NationalElectric Code will require that the labels contain the equipment'sflash protection boundary, its incident energy level, and therequired personal protective equipment (PPE).

Companies will be cited and fined for notcomplying with these standards.


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ARC FLASH ANALYSIS

Arc Flash is the result of a rapid release of energy due to an arcingfault between a phase bus bar and another phase bus bar orneutral or a ground.

Arc flash is the ball of fire that explodes from an electrical shortcircuit.

Arc flash temperatures can easily reach 14,000 to 16,000 degreesF (7760 to 8871 degrees C).

The heat generated by the high current flow melts or vaporizes thematerial and create an arc.


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ARC FLASH ANALYSIS

During an arc fault the air is the conductor.

An arc fault is similar to the arc obtained during electric weldingand the fault has to be manually started by something creating the

path of conduction or a failure such as a breakdown in insulation.

The explosion in arc flash includes a ball of fire and molten metalas well as a pressure force or blast


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ARC FLASH ANALYSIS

The unfortunate aspect of arc flash is that many times, there is aperson standing in its path.

The pressure blast can be enormous, enough to knock a personacross the room, that can blow away doors and covers andmaterial and thus arc flash can

Blind a person or damage his hearing andknock people and things out of its way.

Arc faults are generally limited to systems where the bus voltage isin excess of 120 volts.

Lower voltage levels normally will not sustain an arc.


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ARC FLASH ANALYSIS

Second degree burns occur if the temperature of human skin israised to 175 degrees F (79.44 Degree C) for 0.1 seconds.

A second degree burn, although painful, is considered curable.

This amount of energy (1.2 cal/cm^2) can be compared to holdingyour hand several inches above a disposable lighter.


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ARC FLASH ANALYSIS

Even if the fault is cleared in a shorter period of time, if a workersclothing catches fire or melts to his skin, the time limit of theexposure could be several minutes if not longer.

Depending on the material, clothing may ignite when temperaturesreach between 700 and 1400 degrees F (i.e 371 to 760 degreesC).

If clothing and equipment are worn to limit the exposure of the

worker to values below those identified above, the worker shouldwalk away from an accident, with minimal injury.


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ARC FLASH ANALYSIS

The intent of an arc flash hazard analysis is to

First determine the amount of personal protective equipment(PPE) required by the worker to limit any burn to a seconddegree burn

and

Second, to determine the safe distance away from energizedequipment for unprotected persons.


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ARC FLASH ANALYSIS

As a frame of reference for incident energy, an exposure of 1.2cal/cm2, that can produce the onset of second degree burn to theskin, is used by standards as the benchmark that definesadequate protection against the thermal effects of arc flash.

Limiting the incident energy exposure at the skin surface to nomore than 1.2 cal/cm2 means you can still receive some burninjury, however the primary objective of arc flash protection is to

minimise the injury and probability of death.


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ARC FLASH ANALYSIS

85+% of all Electrical issues begin as Phase to Ground Faults


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ARC FLASH ANALYSIS


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ARC FLASH ANALYSIS

NFPA 70E states that an arc flash hazard is a dangerous condition

associated with the release of energy caused by an electric arc.

Electrical Arcing is caused by:

Human ErrorLoose Connections

Insulation Failure

Poorly Maintained Equipment

Voltage Spikes

Delayed Short Circuit Interruption

Animals


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ARC FLASH ANALYSIS

The Arc flash protection and mitigation by design includes:-

High resistance grounding for phase to ground fault.

Fast arc detection for phase to phase faults

One of the key solution to mitigation is reduction of the time ofdetection and isolation


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ARC FLASH ANALYSIS

High Resistance Grounding (HRG)

Phase to ground faults are most likely and HRG Systems limitsthe fault current so that no arc flash hazard exists.

As per NFPA70E - HRG is a technique available to reduce theArc Flash Hazard of the System

Fast arc detection and tripping for phase to phase faults

For phase to phase faults where arcing is initiated fast detectionand tripping will reduce the arc energy and thus reduce thehazard risk category

Reduction of the time = Reduces the damage= Reduces Hazard cal/cm2

= Reduces Hazard Risk


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ARC FLASH ANALYSIS

In order to understand the hazards associated with an arc flashincident, its important to understand the difference between anarcing short circuit and a bolted short circuit.

Bolted faults (low impedance and high current)

Arcing faults (high impedance, lower current)

A Bolted fault is a theoretical value of the greatest magnitude ofcurrent that can be conducted through the electrical system whenall three phases are shorted together.

Engineers and designers of electrical systems should ensure that

all interrupting devices are rated for this bolted fault capacity.


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ARC FLASH ANALYSIS

The occurrence of a bolted fault is extremely rare without humanor divine intervention.

Some times an arcing fault can occur spontaneously when no oneis present in the area.

Most times, however, arcing faults are caused by some humanintervention, such as

careless cover removal,or dropping a tool into the equipment,

or measuring the voltage.


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ARC FLASH ANALYSIS

In summary, bolted faults are commonly caused byImproper connections after maintenance

Installation errors

In summary, Arcing faults are commonly caused by

Careless cover or device removal

Foreign object (tool) dropped into equipment

Misalignment of moving contacts (parts failure)

Dirt contamination or dielectric breakdown

Entry of foreign body (rodent, snake, squirrel)


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ARC FLASH ANALYSIS

The arc fault current is usually much less than the available boltedfault current.

Unless the protection devices have been selected to handle thearc fault condition, they will not trip and the full force of an arc flashwill occur.

The electrical equation for energy is volts x current x time.

The transition from arc fault to arc flash takes a finite time,increasing in intensity as the pressure wave develops.

The challenge is to sense the arc fault current and shut off thevoltage in a timely manner before it develops into a serious arcflash condition.


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ARC FLASH ANALYSIS

Arcing fault incident energy produced is

Greater at higher bolted fault current levels

Reduced by dynamic impedance (air)

And increased by the time duration of the arc

The most controllable factor in reducing the incident energy, asdiscussed earlier, is time

Fuses or circuit breakers are the first line of defense in reducingarcing fault incident energy


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ARC FLASH ANALYSIS

NFPA70E addresses worker safety and work practicesOSHA is citing NFPA70ENFPA70E will be referenced in work injury litigation

Requirements for compliance:Perform Arc Flash Hazard AnalysisLabel electrical equipment designating the required PPETrain workers and update work practice proceduresDeploy products, solutions, and methods to limit arc flashhazards whenever possible HRGReduce PPE requirements by ARC Flash Protection


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ARC FLASH ANALYSIS

Flash hazard analysis should be done before a person approachesany exposed electrical conductor or circuit part that has not beenplaced in an electrically safe work condition. (NFPA 70E, Part II, 2-1.3.3)

Desired output of arc flash analysis for each equipment includes:

Flash protection boundary distance

Incident energy

Hazard / risk category for Personal Protection Equipment(PPE) selection


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ARC FLASH ANALYSIS

Working Distance

Typical working distance is the sum of the distance between theworker standing in front of the equipment, and from the front of theequipment to the potential arc source inside the equipment.

Arc-flash protection is always based on the incident energy level

on the person's face and body at the working distance, not theincident energy on the hands or arms.

Typical working distances are shown in table 3 of IEEE 1584


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ARC FLASH ANALYSIS

Working Distance

One of the issues that often comes up when performing arc flashcalculation is that of the working distance.

IEEE 1584 provides recommended working distance for use in itscalculations, but in real life, this cannot be precisely followed.

A change of just a few inches can make a tremendous differencein the incident energy received by the worker.

This fact can be used when the incident energy is too high foravailable PPE by increasing the distance from the component orpart to the worker.


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ARC FLASH ANALYSIS

Working Distance


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ARC FLASH ANALYSIS


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ARC FLASH ANALYSIS-PROTECTION BOUNDARIES DIAGRAM


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ARC FLASH ANALYSIS

As you can see from previous slide diagram from NFPA 70E, thereare a number of approach boundaries to be aware of.

Most of these relate to shock one boundary which is the outermostboundary on this diagram is the flash protection boundary.

All these protection boundaries are determined by NFPA 70ETable 2-1.3.4 (2000 Addition) and is based upon the voltage of theequipment


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ARC FLASH ANALYSIS

Restricted Approach Boundary:-

A shock protection boundary to be crossed by only qualifiedpersons (at a distance from a live part) which, due to its proximityto a shock hazard, requires the use of shock protection techniquesand equipment when crossed.

To cross the Restricted Approach Boundary into the RestrictedSpace, the qualified person, who has completed required training,must wear appropriate personal protective equipment (PPE).

Also, he must have a written approved plan for the work that theywill perform and plan the work to keep all parts of the body out ofthe Prohibited Space.

.


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ARC FLASH ANALYSIS

Limited Approach Boundary:-

A shock protection boundary to be crossed by only qualifiedpersons (at a distance from a live part) which is not to be crossedby unqualified persons unless escorted by a qualified person.

The limited approach boundary is the minimum distance from theenergized item where unqualified personnel may safely stand.

No untrained personnel may approach any closer to the energizeditem than this boundary.

A qualified person must use the appropriate PPE and be trained toperform the required work to cross the limited approach boundaryand enter the limited space.


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ARC FLASH ANALYSIS

Flash Protection Boundary Dfb


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ARC FLASH ANALYSIS

The flash protection boundary is at a distance at which incidentenergy is 1.2 Cal/cm2, that is the distance at which a just curableburn would occur.

It does not mean that those outside of the boundary would not be

injured.

It means that the injuries would probably be curable injuries.


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ARC FLASH ANALYSIS

The amount of energy impressed on a surface, a certain distancefrom the source, generated during an arc event.


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ARC FLASH ANALYSIS

In line with IEEE 1584 the list of the steps in an arc flash analysisare as follows:-

Collect system and installation data

Determine system modes of operation

Determine bolted fault current

Determine arc fault currentFind protective device characteristic and arc duration

Document system voltages and equipment class

Select workingdistances.


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ARC FLASH ANALYSIS

Collection of system and installation data

Begin with single-line diagrams and people who know siteCollect information needed for short circuit andcoordination study

Determine each operation mode and perform calculationfor each (maximum fault may not be worst case)

Modes

Multiple feeders

Multiple transformers with tie

Generators in parallel or standby

.


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ARC FLASH ANALYSIS

Determination of bolted fault current:-

Start with utility information

Use information from Steps 1 & 2

Calculate accurate 3-phase bolted fault

High or low values may not give proper arc flash output

Calculation spreadsheet available with IEEE 1584


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ARC FLASH ANALYSIS

Use calculated arc current instead of bolted fault current todetermine duration.

Notice dramatic change below the instantaneous level.Include delay plus device operating time where relay is involved.


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ARC FLASH ANALYSIS

Personnel Protection Equipment (PPE) Hazard Risk Category


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ARC FLASH ANALYSIS

FR = Flame resistant or flame retardant per NFPA 70EPart II, 3-3.2 and 3-3.9.7.2

Hard hat, switching hood and ear protection required for Cat. 3and above.

Once you know the incident energy, it is possible to determine the

minimum Arc Thermal Performance Exposure Value rating ofPPE.


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ARC FLASH ANALYSIS

This fabric is designed to provide a thermal barrier and limit theincident energy exposure at the skin surface to no greater than 1.2

cal / cm2.

Although FR fabric will burn when exposed to a flame, it isdesigned to stop burning when the flame is removed.

It also must not break or burn open and expose the skin directly tothe flame.

To properly protect a worker, the ATPV rating of the FR clothing

must exceed the prospective incident energy available at a givenlocation.


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ARC FLASH ANALYSIS

Analogy Of Safety aspects of electrical industry as compared withautomotive industry


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ARC FLASH ANALYSIS

Analogy Of Active Safety aspects of electrical industry ascompared with automotive industry


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ARC FLASH ANALYSIS

Statistics show that 90% of all Faults start as ground faults.

One disadvantage of the solidly grounded 415 V system involvesthe high magnitude of destructive, arcing ground-fault currents thatcan occur.

A safety hazard exists for solidly grounded systems from thesevere flash, arc burning, and blast hazard from any phase-to-ground fault.

However, if these currents are promptly interrupted, the equipmentdamage is kept to acceptable levels.


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ARC FLASH ANALYSIS

Effect of arc current variation on determination of clearing time

For protective devices operating in the steep portion of their time-currentcurves, a small change in current causes a big change in operating time.

Incident energy is linear with time, so arc current variation may have a bigeffect on incident energy.

The solution is to make two arc current and energy calculations; one usingthe calculated expected arc current and one using a reduced arc currentthat is 15% lower.

It requires that an operating time be determined for both the expected arccurrent and the reduced arc current.

Incident energy is calculated for both sets of arc currents and operatingtimes and the larger incident energy is taken as the result.


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ARC FLASH ANALYSIS

Although much research has already been conducted in the area

of arc flash, much is yet to be learned.

There is presently a joint collaborative effort underway betweenthe NFPA and IEEE that will take arc flash research to the nextlevel.

This multi-year effort has a budget of over US $6m and willattempt to answer questions such as how to calculate

DC arc flash,

blast pressure,arc sustainability and much more.

Documents

A- Arc Flash Analysis-general